Only after the gameplay ends, the server hashed string+salt influenced by the client seed will be compared with the player pick and the outcome will be decided and the salt will be revealed.
How is the outcome influenced by the client seed? Is that written somewhere?
If the algorithm that converts (server string + salt + client seed) into a game outcome is secret, then the game isn't provably fair. Because the algorithm could be "we often deliberately pick a result that makes you lose" and the players wouldn't be able to tell.